History, LQ4 with 799 heads, LS6 intake WS6 store high lift LSA cam, long tubes with X pipe. I've had it tuned and it runs great but after about ten minutes I get these two codes. It still runs fine. I hooked up my scan tool and the O2 sensors are reporting .420-.455 idling and occasionally lower. They are new delco sensors. I have searched all over and see a lot of these threads but naturally most of them just die with no answers to what fixed the issue.

 

Those codes mean the front O2s have no activity. The data you see, only cycling 420 to 455 mV, shows they aren't moving. They should be cycling from around 100mv to over 900mV, once or twice a second. They might sit at 450mV until they warm up, but that should only take a minute or so. After that they should be cycling.
If they are new double check that they are wired correctly.
Joe

 

Looks like my brand new Delco bank 2 sensor was internally shorted. I checked for power to the sensors and had none so I unplugged the harness and checked for power leaving the PCM and had 12 volts. I was baffled for a minute and then went back under the car and had power on both plugs while unplugged. I plugged in one at a time and had power with bank 1 plugged in but lost power as soon as I plugged in bank 2. I know the power comes from the PCM and its not fused but it must be protected internally some how. Anyway its fixed and no more aggravating CEL. Thanks for the help guys.

 

Well it looks like I was wrong. Its not fixed. I drove it a little farther today and got my codes back. So today has been diagnostic work. I rechecked the circuit and have power without the 02 sensors plugged in and immediately lose it when I plug any of four sensors that I have in. I have checked the heater circuit in all four sensors and they ohm out good. I have the odd 2001 6.0 that gets 02 power from pin 74 on the ECM so I pulled the ECM and checked and its loosing power at the pin when I plug up a sensor. So??? Bad solder joint in the ECM? During the harness rework process I learned that most 2001 and 2002 trucks supplied power directly to the sensors with switched battery power. Can I eliminate pin 74 and wire my sensors to keyed power? Cruise season is almost here and I'm ready to ride.

 

Well I know the cause now just have no clue how to fix it. Before going to the tuner I bought two new O2 sensors and installed them. I have the odd ball ECM with isolated grounds and power to the sensors from the ECM. Good old Oreilly sold me 02 sensors for the same year and the same plug but they have case grounds. So that has apparently damaged the circuit in my ECM. Its still will show power on that circuit until any load is applied then it shows none. Am I posting this in the correct location?

 

Are you sure you smoked the PCM Driver? I'm not aware of an LQ4 that has O2 power supplied by the PCM.
What year/make/model is the engine from?
Joe

 

2001 GMC Yukon Denali 411 PCM. Brendon shows the diagram and pin outs on his 02 section on lt1swap.com. With no load the circuit shows 12 volt. As soon as I plug in the sensors that came with the Denali the reading drops to 0. My question is can I supply the 02 sensors with switched power and still use the same PCM? Basically not use pin 74 to power the sensors.

http://lt1swap.com/01_6.0_isolated.gif

 

Well that's a new one on me! I've never seen a GM vehicle of that vintage with anything but fused O2s, like the rear ones. Interesting.

Unplug the O2s until power comes back on the harness. Will the 12V coming from the PCM power a regular 12V test light? If it is powering O2 sensors it should be able to flow at least 10 amps. If it will light the test light, try plugging the O2s back in one at a time. Maybe you have a shorted heater pulling the circuit to ground. There's a good chance that driver circuit is current protected. You may not have smoked it. If you did, it's free to try powering the O2s with fused 12V. The PCM would already be damaged, so it would be no harm in trying.

Does your scanner measure front O2 heater amps? If the PCM controls power to them, like on newer vehicles, it should display it. That info might help to know whether or not you'll get away with fused 12V.

Joe

 

I'll try the old style test light. I have tested the heater circuit on the sensors outside of the car using a battery. They both drew around two amps and heated up as expected. The sensors that came in the Denali have a heater circuit that is totally isolated from the case and the two I purchased both have one of the wires directly connected to the case of the sensor. I'll also run a 02 monitor test using my scan tool. Worst case if powering them independently throws codes I can pick up a replacement PCM. Would the tuner have my latest tune stored on his laptop so it can be loaded into a replacement PCM?

 

I just reinstalled the ECM and with no 02 sensors plugged up I have 12.7 volts on pin 74. It will not light an old school test light. As soon as it makes contact the power disappears. So if I wire the hot and grounds of the 02 sensors what exactly will the ECM no be able to read on pins 74 and 80? Or should I go pick up a replacement PCM and have the shop that tuned my engine swap the tune over to the replacement? Or can he even do that?

 

I'm not sure we will get an answer to this unless someone has already tried it, or unless you can find some documentation on why they use this setup.
On any other GM setup where the PCM controls the heaters, they are ground side switched. They turn them on by applying ground, and measure the O2 heater current on that same pin. If there is any current limiting to be done, it's done on the ground side.
In your case, maybe they current limit the circuit on the B+ Side. Maybe the measure the current there too. I don't know.
If that pin can't power a test light, it sure can't power the O2s, so I say it's probably smoked. If you had O2s that were floating ground, it would be free to try putting fused power to the B+ side. However, your O2s are grounded all the time, so they'll probably pop the heaters before too long. I think you're gonna need the correct O2s and a new PCM.
The tuning question can only be answered by your tuner. There's too many variables.
My 2 cents. (actual street value)

 

Actually the 4.8 and 5.3 in 2001-2002 used the same pcm and had the 02 sensors powered directly from switched power and grounded to chassis ground with no regulation. I'm losing faith in my tuner, he admitted last night that he had noticed that the 02 sensors were not working when he tuned and he tuned it in open loop to ignore the sensors. He did say that if I fixed my 02 issue he would go back over it and tune it closed loop. I'm new to the new injected engines and not sure where to go from here.

http://lt1swap.com/02_4.8-5.3_isolate_ground.gif

 

It's uncharted territory. If it was mine, I'd try powering the O2s with a fused 12V wire. Then I'd check to see if the O2s are working. They should be reading something other than 450mV in under a minute. If they do, see if they live for 5 minutes. If the heaters make it that long I'd say you're in business.
Check Pending Codes to make sure.
You're quite a ways from my house, but it you feel like driving out to Greensboro area we can fool with it.

Joe

 

My buddy that I work with is from Kernersville. My plan to keep from butchering my harness is to run a switched power and ground to the sensors and connect the switched power but leave the ECM ground and sensor wires intact and try that. If I have to use the chassis ground it will already be in the loom . I have had the sensors on a battery for five minutes or so already and they heat up and level off and lived. It should work unless the ECM is looking for feedback on pin 74. If that works I'll head back to the tuner for a closed loop tune.

 

I tied powering the 02 sensors and they heated up and did not burn up but still no reporting to the ECM which tells me that I have ben wrong all along. I didn't hurt my ECM, the tuner caused this. Apparently he was having difficulties with the tune and took It upon himself to turn off the front 02 sensors and did a open loop tune but he did not disable the CEL for the sensors and never mentioned it to me. The car runs great and I have amazing power but I'm not happy that he did what he did. I'm going to take it back to him to have those two codes disabled but in the long run I'm considering the new Holley Terminator X MAX system.